1. Field of the Invention
The present invention relates to a process for preparing a thin film of Bi-type oxide superconductor, more particularly to a process for depositing a single crystal thin film of Bi-type oxide superconductor whose c-axis is oriented in parallel with a surface of a substrate.
2. Description of the Related Art
Oxide superconductors of the Bi-Sr-Ca-Cu-0 system (hereinafter, Bi-type oxide superconductors) possess higher critical temperatures than those of the Y-Ba-Cu-0 system and hence are expected to have many practical applications.
In the meantime, it is indispensable to prepare thin films of such oxide superconductors in order to actually utilize them in electronic devices such as Josephson devices and superconducting transistors. Today, it is possible to prepare a thin film of single crystal of an oxide superconductor of the Y-Ba-Cu-O system by a variety of techniques including sputtering, vacuum evaporation, laser abeation, or the like.
However, in the case of Bi-type oxide superconductors, it is difficult or impossible to prepare a single crystal thin film having a well-ordered crystalline structure for several reasons; in particular, because more than two phases, each having a different superconducting property, are produced simultaneously.
It is known that the Bi-type oxide superconductors show anisotropy in their superconducting properties. In fact, their crystals show higher critical current density (Jc) along a direction which is perpendicular to the c-axis than along the other directions. In many superconducting devices, at least one additional layer is deposited on the thin film of Bi-type oxide superconductor. In such devices, the thin film of Bi-type oxide superconductor is required to have a crystal orientation wherein the c-axis of the thin film is oriented in parallel with the surface of the substrate in order to obtain higher critical current flow in a direction (depth-wise direction) which is perpendicular to the surface of the substrate.
However, the c-axis of thin films of Bi-type oxide superconductors prepared by known techniques is oriented perpendicularly to the surface of the substrate. In other words, it is difficult to prepare a single crystal thin film of Bi-type oxide superconductor whose c-axis is oriented in parallel with the surface of substrate, so that thin films of Bi-type oxide superconductor prepared by known techniques have not been utilized advantageously in superconducting electronic devices.
Furthermore, thin films of Bi-type oxide superconductors prepared by known techniques do not have a smooth surface and show relatively poor superconducting properties and hence cannot be practically used.
Still further, the conventional processes cannot stably or reproducibly produce thin films of Bi-type oxide superconductors having uniform properties.
An object of the present invention is to solve the problems and to provide a process for preparing a single crystal thin film of Bi-type oxide superconductor whose c-axis is oriented in parallel with the surface of a substrate.